Conventionally, a number of methods for polymerizing or copolymerizing olefins in the presence of a catalyst for olefin polymerization comprising a solid catalyst component containing magnesium, titanium, an electron donor compound, and halogen as essential components, an organoaluminum compound, and an organosilicon compound have been proposed.
For example, Patent Document 1 (Japanese Patent Application Laid-open No. 63310/1982) and Patent Document 2 (Japanese Patent Application Laid-open No. 63311/1982) propose a process for polymerizing olefins with three or more carbon atoms, in which a catalyst comprising a combination of a solid catalyst component containing a magnesium compound, titanium compound, and an electron donor, an organoaluminum compound, and an organosilicon compound having a Si—O—C bond is used. However, because this process is not necessarily satisfactory for producing highly stereoregular polymers in a high yield, further improvement of the process has been desired.
Patent Document 3 (Japanese Patent Application Laid-open No. 3010/1988) proposes a catalyst for olefin polymerization and a process for polymerizing olefins in the presence of the catalyst. The catalyst for olefin polymerization comprises a solid catalyst component, prepared by heat-processing of a powdered product obtained by causing dialkoxymagnesium, aromatic dicarboxylic acid diester, aromatic hydrocarbon, and titanium halide to come in contact with each other, an organoaluminum compound, and an organosilicon compound.
Patent Document 4 (Japanese Patent Application Laid-open No. 315406/1989) proposes another catalyst for olefin polymerization and a process for polymerizing olefins in the presence of this catalyst. The catalyst for olefin polymerization comprises a solid catalyst component prepared by causing a suspension containing diethoxymagnesium and alkylbenzene to come in contact with titanium tetrachloride, reacting the suspension with phthalic acid dichloride, and causing the resulting solid product to come in contact with titanium tetrachloride in the presence of alkylbenzene, an organoaluminum compound, and an organosilicon compound.
Patent Document 5 (Japanese Patent Application Laid-open No. 84404/1990) proposes a catalyst for olefin polymerization comprising a solid titanium catalyst component containing magnesium, titanium, and halogen as the essential components obtained by causing a magnesium compound and a titanium compound to come in contact with each other, an organoaluminum compound catalyst component, and an organosilicon compound catalyst component containing a cyclopentyl group, cyclopentenyl group, cyclopentadienyl group, or a derivative of these groups, as well as a process for polymerizing or copolymerizing olefins in the presence of this catalyst.
All of the above-described conventional technologies have attained excellent results in improving the catalyst activity to enable omission of an ash-removal step for removing catalyst residues such as chlorine and titanium from the formed polymers, improving the yield of stereoregular polymers, and improving the durability of the catalyst activity during the polymerization.
However, olefin polymers obtained by polymerizing olefins using a catalyst for olefin polymerization containing this type of highly active catalyst component, an organoaluminum compound, and an organosilicon compound have been found to possess a molecular weight distribution narrower than that of olefin polymers obtained by polymerizing olefins using a catalyst for olefin polymerization in which the conventional titanium trichloride catalyst component is combined with an organoaluminum compound and, optionally, an electron donor compound as the third component. The narrow molecular weight distribution leads to a low melting viscoelasticity of the polymer to be processed, which gives rise to an impaired outward appearance of the final products (polyolefins). The application of the olefin polymers must be limited to a certain degree.
Various ideas have been put into practice in an attempt to solve this problem. One example is using multi-stage polymerization to obtain polyolefin with a broad molecular weight distribution. However, the multi-stage polymerization involves undesirable features including a high cost such as repetition of a complicated polymerization process and reclaiming of a chelating agent used in the polymerization.
Patent Document 6 (Japanese Patent Application Laid-open No. 7703/1991) proposes a process for polymerizing olefins in the presence of a catalyst for olefin polymerization comprising a solid titanium catalyst component containing magnesium, titanium, halogen, and an electron donor as essential components, an organoaluminum compound, and at least two electron donors (organosilicon compounds).
The applicants claim that the target polyolefin with a broad molecular weight distribution can be obtained without requiring a complicated multi-stage polymerization. However, the requirement of using two or more organosilicon compounds as electron donors during polymerization makes the process complicated.
Accordingly, an object of the present invention is to solve the above problems remaining in the prior art and to provide a catalyst for olefin polymerization and a process for polymerizing olefins, which can produce olefin polymers having a broad molecular weight distribution by a simple procedure while maintaining high stereoregularity.